Cor Caroli or Alpha 12 Canum Venaticorum (what a mouthful!) is a superb double star in the constellation of Canes Venatici (The Hunting Dogs). This beautiful system is about 120 light years away and the two stars are separated by 770 A.U. (Astronomical Units) which is roughly the width of 5 solar systems. The primary star shines at magnitude 2.9 with the secondary star at magnitude 5.5 and is a superb double for 2 to 3 inch telescopes.
Apart from the Latin, this star had no proper name until the astronomer Edmund Halley designated it Cor Caroli in the 17th century. The name Cor Caroli actually translates as "Heart of Charles" and Halley named it in honour of King Charles I of England, who was beheaded under the rule of Oliver Cromwell. This double is best observed with an intact head in the Winter and Spring months, especially March.
These two stars are supposed to appear as white and usually do through most small telescopes. If observed through a 8 inch telescope or larger the blue and green tinge is apparent. However, if you look carefully at my close up photo above taken through a 4.75 inch refractor, you can clearly see that the primary is tinged blue and the secondary is green. Now, this is a problem as there are no green stars in reality. So, what's happening here? Well, the general theory is that the closeness of the stars from Earth's proximity means that the brighter primary is causing an optical illusion which makes the secondary appear green. Hmm, is science wrong or is it just our poor eyesight? Better get it checked out then!